When playing against human-controlled zombies in Left 4 Dead, their ability to …

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When you're playing the versus mode in Left 4 Dead, the zombie's ability to see the outline of your character through the walls gives them a huge advantage while planning their attacks. There are some things you can do to make that outline disappear so you can move in secret, but what those things are have been subject to debate online. Finally, someone on the Steam forum has outlined exactly what you can and can't do if you want to remain unseen.

I can see you!

The amount of things you can do and remain hidden is impressive, but you have to know what you're doing. Here's the partial list:

The Flashlight! Whether it's on or not is irrelevant. The
flashlight does NOT affect the visibility of your outline. Given that,
there's pretty much no reason to ever turn it off.

Walking. If you hold shift while moving, you walk instead of
running. This is slower, of course, but your outline remains invisible
while you walk. Good if you want to reverse an ambush by snaking up on
a hunter.

Crouch walking. Moving while holding crouch (ctrl) is exactly like walking. Holding the walk key while crouched does nothing.

Releasing a grenade throw that you had ready. This is covered above.

Reloading your weapon. Doesn't make you visible, but sometimes your
survivor will shout "reloading!" and voice clips do make you visible,
so beware of that.

Healing. As with reloading, using a medkit itself doesn't highlight you, with the caveat that the voice clips can.

Be sure to read the whole post for the full lists of things that will reveal your position and actions that won't. Knowing these inside and out will give you a significant advantage while playing online; I'm glad someone finally went through the trouble of testing all these actions. Well done!